Apparatus and method for optimizing the air inlet temperature of gas turbines

ABSTRACT

A heat exchanger is disclosed for heating air entering a combustion gas turbine to increase the power output of the turbine when the turbine operates in a cold environment. The heat exchanger may be used also as a cooler to cool air entering the turbine to increase the power output of the turbine when the turbine operates in a hot environment.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to the field of combustion gas turbines,and more particularly, to a method and an apparatus for optimizing theinlet temperature of the air flowing to a combustion gas turbine toimprove the performance thereof. Still more particularly, the presentinvention discloses a method and an apparatus for raising the inlettemperature of the air when the ambient temperature is low to increasethe power output of the combustion gas turbine in a cold environment.The apparatus may be used, not only as a heater to raise the inlettemperature of the air when the turbine is operated under coldconditions, but also, it may be used as a cooler to decrease the inlettemperature of the air to increase the power output of the turbine whenthe turbine is operated under hot conditions. Furthermore, the apparatusmay be used in connection with a gas turbine as a heater only togetherwith a separate cooler, the heater being placed into operation when theambient temperature is low and the cooler being placed into operationwhen the ambient temperature is high.

BACKGROUND OF THE INVENTION

Combustion gas turbines are well known in the art. In general, thoseturbines include a compression section for compressing air entering theturbine, a combustion section following the compression section wherethe compressed air is combusted with fuel, and an expansion section,following the combustion section, where the combustion mixture from thecombustion section is expanded to generate shaft work. The shaft work istransferred to an outside user that utilizes such shaft work. In manyapplications, the shaft work is transferred to an electrical generatorthat transforms the shaft work to electricity. The hot exhaust from theexpansion section flows to a waste heat recovery unit where heat isrecovered by generating steam or by providing heat to other media orheat utilizers.

Combustion gas turbines are constructed as single, double or tripleshaft turbines Single shaft turbines include only one shaft utilized byboth compression and expansion section at the same speed. A double shaftturbine includes two shafts, one shaft transferring work from theexpansion section to the compression section and another shafttransferring work from the expansion section to a driven load. A tripleshaft turbine includes one shaft transferring work from the expansionsection to a portion of the compression section, a second shafttransferring work from the expansion section to another portion of thecompression section, and a third shaft transferring work from theexpansion section to the driven load. Although, single shaft turbineswere used more often to generate work in the past, the use of double andtriple shaft turbines has recently increased.

Several factors affect the performance and the work output generated bycombustion gas turbines. One major factor is the inlet temperature ofthe air entering the compression stage of the turbine. Its effect on thepower output of the gas turbine depends on the number of shafts of saidturbine. In single shaft turbines, the output increases in asubstantially linear fashion until it reaches a plateau as the inlet airtemperature decreases. This correlation results from the fact that asthe inlet temperature decreases, the density of the air increaseswhereby a larger mass of air flows through the turbine to generate anincreased amount of work. FIG. 1 shows the above correlation ofelectricity generated versus inlet temperature of air for a single shaftcombustion gas turbine operating with natural gas fuel at sea level,sixty (60) percent relative humidity, 60 hz, inlet loss of 4 inches ofH₂ O, exhaust loss of 10 inches of H₂ O, and with no steam or waterinjection for control of nitrogen oxides emissions. The abscissa showsthe inlet temperature of the combustion air in degrees Fahrenheit (°F.)and the ordinate shows the output at the generator terminals inkilowatts (kw).

In multi-shaft, i.e., double or triple shaft gas turbines, thecorrelation between output and inlet air temperature is different inthat, although the output increases as the air inlet temperaturedecreases in a particular temperature range, the output reaches amaximum at the lowest point of that range and decreases as thetemperature decreases below that point. Referring now to FIG. 2, thereis shown a graph depicting the correlation between electrical outputversus air inlet temperature of a double shaft General Electric LM2500gas turbine generating electricity and operating with natural gas fuelat sea level, sixty (60) percent relative humidity, 60 hz, inlet loss of4 inches of H₂ O, exhaust loss of 10 inches of H₂ O, and with waterinjection for control of nitrogen oxides emissions, the amount of thewater being sufficient to meet the typical regulatory emissionrequirements of nitrogen oxides of about 42 parts per million on a drybasis. The abscissa shows the inlet temperature of the combustion air indegrees Fahrenheit (°F.), and the ordinate shows the output at thegenerator terminals in kilowatts (kw). FIG. 2 shows that the electricaloutput increases from about 18,500 kilowatts to about 24,300 kilowattsas the inlet temperature of the air decreases from 100° F. to 35° F. Asthe temperature decreases below 35° F., the electrical output decreaseswith such temperature decrease. Therefore, it appears from FIG. 2 thatthe most desirable air inlet temperature for that particular turbine isabout 35° F.

Triple shaft gas turbines have a similar maximum electrical outputachieved at a particular air inlet temperature. Referring now to FIG. 3,there is shown a graph depicting the correlation between electricaloutput and inlet temperature of air in a triple shaft General ElectricLM5000 gas turbine generating electricity and operating with natural gasfuel at sea level, sixty (60) percent relative humidity, 60 hz, inletloss of 4 inches of H₂ O, exhaust loss of 10 inches of H₂ O, with steaminjection for control of oxides of nitrogen emissions (about 42 partsper million on a dry basis), and additional steam injection for poweraugmentation. The abscissa shows the inlet temperature of the combustionair in degrees Fahrenheit (°F.), and the ordinate shows the output atthe generator terminals in kilowatts (kw). There is shown that theelectrical output increases from about 39,500 kilowatts to about 53,000kilowatts as the temperature decreases from 100° F. to 40° F. Theelectrical output starts decreasing beyond that point (40° F.) as theinlet temperature of the air decreases. Therefore, it is apparent thatit is desirable to operate the gas turbine with an air inlet temperatureof about 40° F.

In the past, because gas turbines have been more commonly used togenerate power in hot climates, only coolers have been used to decreasethe inlet temperature of the air to increase the power output. Heatershave not been used to increase the air inlet temperature towards theoptimum air inlet temperature, as demonstrated by the above graphs, toincrease the power output towards its maximum. As a result, themultishaft gas turbines previously used in cold environments did notproduce the maximum output achievable by those turbines.

According to the present invention, a method and an apparatus aredisclosed to increase the inlet temperature of the air in cold climatesto obtain the optimum air inlet temperature by heating the air in aheater. The heater may be the same apparatus that is used to cool theair to reach the optimum air inlet temperature when the ambienttemperature is high due to hot weather conditions. In those instances,the apparatus is sometimes referred to herein as the heater/cooler. Theheater may also be a separate apparatus which is operated only duringthe cold periods while a separate cooling apparatus is used alone duringthe hot periods.

Another problem encountered in the past in cold climates has been theformation of ice at the inlet of the gas turbine caused by thecondensation of water thereon. The accumulation of such ice isoftentimes very rapid and causes plugging of the filter surface,possible engine damage from ice formed at the engine bellmouth, and atotal shutdown of the gas turbine. In the past, this problem has beenaddressed by flowing hot exhaust gases from the outlet of the turbinethrough a heat exchanger and over the inlet thereof to prevent suchicing. One disadvantage of that method was that it required the additionof special equipment such as jackets around the inlet. Anotherdisadvantage was that the hot gases were available at substantially hightemperatures whereby they formed hot spots around the inlet of theturbine. Still another disadvantage was that the temperature at theinlet of the gas turbine could not be easily controlled. The addition ofthe heater disclosed by the present invention prevents the formation ofice at the inlet of the gas turbine while eliminating the problems ofprevious deicing techniques.

These and other advantages of the present invention will become apparentfrom the following description and drawings.

SUMMARY OF THE INVENTION

A method and an apparatus are disclosed for heating the air entering amulti-shaft gas turbine to increase the output of such turbine in coldclimates. The heater may be the same apparatus used to cool the air toincrease the output of the gas turbine when the turbine is operated in ahot ambient environment. Furthermore, the heater may be a separateapparatus which is used only for heating when the ambient temperature islow while a separate cooler is used to cool the air when the ambienttemperature is high.

In cogeneration plants i.e., when a combustion gas turbine is used togenerate electricity and steam, steam is used to supply heat to aheating medium that flows to the heater to heat the air entering the gasturbine. When the heater is the same apparatus used also as a coolerwhen the ambient temperature is high, the heater/cooler is connected toa cooling source that provides cooling to a cooling medium passingtherethrough when cooling, rather than heating, is required. The coolingsource also utilizes steam available in the cogeneration plant. Otherheating and cooling sources may also be utilized depending on the energyavailability in the particular operation.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of the embodiments of the apparatus and themethod of the present invention, reference will now be made to theaccompanying drawings wherein:

FIG. 1 is a graph showing the electrical output of a typical singleshaft gas turbine generating electricity in kilowatts (kw) as a functionof the inlet temperature of the combustion air flowing to such turbinein degrees Fahrenheit (°F.);

FIG. 2 is a graph showing the electrical output of a double shaft gasturbine (General Electric LM-2500) generating electricity in kilowatts(kw) as a function of the inlet temperature of the combustion airentering such turbine in degrees Fahrenheit (°F.);

FIG. 3 is a graph showing the performance of a triple shaft gas turbine(General Electric LM-5000) generating electricity in kilowatts (kw) as afunction of the inlet temperature of the combustion air entering suchturbine in degrees Fahrenheit (°F.); and

FIG. 4 is a flow sheet showing a cogeneration process generatingelectrical power and steam and the method and apparatus of the presentinvention wherein a heat exchanger is used to heat the air entering thegas turbine when the ambient temperature is low and to cool the airentering the gas turbine when the ambient temperature is high.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The power output of a combustion gas turbine is a function of thetemperature of the air flowing into the turbine. In single shaftturbines, the power output of the turbine increases as the inlet airtemperature decreases. In multishaft combustion gas turbines, the poweroutput thereof increases as the air inlet temperature decreases but onlyover a certain temperature range. The power output reaches a maximum atthe lowest point of that range and begins to decrease as the air inlettemperature decreases beyond that point. FIGS. 2 and 3 show thatrelationship. The lowest point of the range in which the power reaches amaximum is hereinafter sometimes referred to as the optimum inlettemperature. Accordingly, when a multishaft gas turbine operates in ahot environment wherein the ambient temperature is above the optimuminlet temperature, it is desirable to cool the air flowing to theturbine towards that optimum inlet temperature to increase its poweroutput. Similarly, when a multishaft turbine operates in a coldenvironment where the ambient temperature is less than the optimum inlettemperature, it is desirable to increase the air inlet temperature up tosuch optimum inlet temperature to increase the power output thereof.

According to the present invention, when a multishaft combustion gasturbine is operated in an environment where the ambient temperature isless than the optimum inlet temperature for the turbine, the air isheated in a heat exchanger by exchanging heat with a heating medium suchas a hot fluid or the like being heated by heat which is available fromthe gas turbine. Preferably, the heat exchanger may be utilized also asa cooler to cool the air entering the turbine when the ambienttemperature is above the optimum inlet temperature to increase theperformance of the turbine. Alternatively, the heat exchanger may beused alone as a heater for applications wherein the ambient temperaturedoes not exceed the optimum inlet temperature or it may be used incombination with a separate heat exchanger which operates as a coolerfor those instances wherein the ambient temperature exceeds the optimuminlet temperature.

Referring now to FIG. 4, there is shown a cogeneration apparatus forgenerating electricity and steam having a heat exchanger for optimizingthe temperature of the air entering a combustion gas turbine inaccordance with the present invention. The heat exchanger is used as aheater to heat the air flowing to the combustion gas turbine when theambient temperature is below the optimum inlet temperature and as acooler to cool the air flowing to the combustion gas turbine when theturbine is operated in an environment where the ambient temperature isabove the optimum inlet temperature. The cogeneration apparatus includesa gas turbine 20 being connected to an electrical generator 22 via shaft24, an air filter/coil module 26 for treating the air inlet to turbine20, a steam generator 28, a heating source 30 and a cooling source 32.Turbine 20 includes a compression section 34 a combustion section 36,and an expansion section 38. Air filter/coil module 26 includes aprefilter 40, a heat exchanger 42, a demister 44, final filters 46 andsilencer 48. An air stream 1 enters air filter/coil module 26 and flows,in sequence, through prefilter 40, heat exchanger 42, demister 44, finalfilters 46 and silencer 48 for preparation prior to entering turbine 20.Prefilter 40 removes large particulates from air stream 1.

Heat exchanger 42 heats or cools air stream 1 in accordance with thepresent invention, depending on the temperature of the air entering airfilter/coil module 26 and the optimum inlet temperature of gas turbine20. More particularly, if the ambient air temperature is less than theoptimum inlet temperature of turbine 20, heat exchanger 42 heats airstream 1 to raise its temperature up to the optimum inlet temperature byexchanging heat between air stream 1 and a hot heating medium providedin stream 11, as hereinafter described. Alternatively, if the ambienttemperature of air stream 1 is greater than the optimum inlettemperature of gas turbine 20, heat exchanger 42 cools air stream 1 toreduce its temperature towards the optimum inlet temperature byproviding cooling from a cold cooling medium provided through stream 11,as hereinafter described. In the event that the ambient temperature ofair stream 1 is about equal to the optimum inlet temperature of turbine20, no cooling or heating of air stream 1 is necessary in heat exchanger42.

Demister 44 removes water that may be entrained in air stream 1, finalfilters 46 remove any fine particles that may be present in said airstream 1, and silencer 48 reduces the noise being generated by the flowof air stream 1.

Upon exiting air filter/coil module 26, air stream enters compressionsection 36 where air stream 1 is compressed. The compressed air flowsinto combustion section 38 where it is combusted by combustion fuel gasflowing therein via combustion gas stream 2 in the presence of waterflowing therein via water stream 3. Water is provided via water stream 3to reduce the nitrogen oxiges emissions to the allowable level. Thecombustion gases being generated flow into expansion section 38 andgenerate shaft work which is transferred via shaft 24 to electricalgenerator 22 to generate electricity. Exhaust gas stream 5 containinghot gases flows from the outlet of turbine 20 to steam generator 28where it heats and vaporizes boiler feed water entering steam generator28 via line 6 to generate saturated high pressure steam exiting throughsteam stream 7 for appropriate utilization.

Condensate which is available from the utilization of steam stream 7 isreturned to the steam generator via stream 8. Makeup water is providedby water stream 9.

Although heat exchanger 42 can be any heat exchanger that is suitablefor heating or cooling an air stream, it is preferred that a coil heatexchanger having coils of finned tube type construction be used. Airflows through the exterior of the tubes and the heating or coolingmedium flows through the tubes.

In the heating mode, i.e., when the ambient temperature is less than theoptimum inlet temperature of turbine 20 and air stream 1 requiresheating to increase the output of turbine 20, a hot heating mediumcomprised of water, glycol or other similar heat transfer media ormixtures thereof flows from heating source 30 to heat exchanger 42 viastreams 10 and 11 to heat air stream 1. The cold heating medium exitsheat exchanger 42 via stream 11 and returns to heating source 30 viastream 12 for further heating and recirculation to heat exchanger 42. Asteam stream 13 supplies steam from steam stream 7 to heating source 30via line 16 to heat the circulating heating medium.

In the cooling mode, i.e., when the ambient temperature is greater thanthe optimum inlet temperature and the performance of turbine 20 can beimproved by cooling air stream 1 towards that temperature, a coldcooling medium such as water, glycol or other similar heat transfermedia or mixtures thereof is circulated from cooling source 32 to heatexchanger 42 through streams 14 and 11 to cool air stream 1. Followingthe cooling of air stream 1, the hot cooling medium exits heat exchanger42 via stream 11 and returns to chiller 32 via stream 15 for furthercooling and recirculation to heat exchanger 42. Cooling source 32 is anabsorption type chiller which utilizes steam from stream 7 flowing tochiller 32 via steam lines 13 and 17.

In the event that the ambient temperature is substantially equal to theoptimum inlet temperature 20, no cooling or heating media are circulatedin heat exchanger 42 via stream 11. The flow of heating or coolingmedium in stream 11 is controlled in response to the temperature of airstream 1 entering turbine 20 and a predetermined optimum inlettemperature. Accordingly, well known flow control techniques andinstruments may be used. Control valves 51, 52, 53, 54, 55, 56, and 57control and regulate the flow in streams and lines 10, 12, 14, 15, 16,17, and 13, respectively.

The following examples further illustrate the invention, but are not tobe construed as limitations on the scope of the process and apparatuscontemplated herein.

EXAMPLE 1

An LM2500 gas turbine manufactured by the General Electric AircraftEngine Group of Evendale, Ohio, was placed in operation in accordancewith the flow scheme shown in FIG. 4 in which the ambient temperature ofair was 80° F. and its relative humidity was 80%. No heat exchange fluidwas circulated in stream 11. The air flowed through air filter/coilmodule 26 with a total pressure drop of about 4 inches of H₂ O andentered compression section 36 of turbine 20 at 80° F. and 80% relativehumidity. The air was compressed in compression section 36 and flowed tocombustion section 38 where it was burned by 207.9 MMBTU per hour oflower heating value gas in the presence of 7205 pounds per hour ofwater. The exhaust gases were expanded in expansion section 38 andexited turbine 20 at 997° F. at a rate of 143.5 pounds per second. Shaft24 transferred work to electrical generator 22 which generated 20,682kilowatts of electricity. Exhaust stream 5 supplied heat in steamgenerator 28 and generated 91,800 pounds per hour of 150 psig saturatedsteam in steam line 7 by vaporizing return condensate returned by stream8 at 200° F. and make up water provided by stream 9. Exhaust gas stream5 exited steam generator 28 at 280° F. No steam was allowed to flowthrough steam stream 13 to either cooling source 32 or heating source30.

EXAMPLE 2

The turbine of Example 1 was placed in operation in the same environmentas Example 1 where the ambient temperature was 80° F. and the relativehumidity was 80%. Air stream 1 was cooled in heat exchanger 42 by watercirculating at 1200 gallons per minute being available at 45° F. Airstream 1 exiting air filter/coil module 26 was cooled to 61.9° F. and arelative humidity of 99%. The cooling water exiting heat exchanger 42was at 55° F. and was returned to cooling source 32 for further coolingand/or circulation. The cooled air stream 1 was compressed incompression section 34 and combusted in combustion section 36 by 223.5MBTU per hour of low heating value gas in the presence of 8,738 poundsper hour of water which was injected for control of nitrogen oxidesemissions. The shaft work generated in expansion section 38 wastransmitted to electrical generator 22 via shaft 24 and generated 22,637kilowatts of electricity. Exhaust stream 5 exited turbine 20 at 981° F.at a rate of 152.2 pounds per second. Exhaust stream 5 generated 85,600pounds per hour of 150 psig saturated steam in steam generator 28 byvaporizing condensate return provided at 200° F. by stream 8 and make-upwater provided by stream 9. Exhaust stream 5 exited steam generator 28at 285° F. The circulating water in stream 11 was cooled in coolingsource 32 from 55° to 45° F. by an absorption process utilizing 9000pounds per hour of steam available from steam stream 7 through steamlines 13 and 17. Example 2 shows that the cooling of the air inlet toturbine 20 from 80° F. to 61.9° F. increased the output of electricgenerator 22 from about 20,682 to about 22,637 kilowatts.

EXAMPLE 3

The turbine of Example 1 was placed in operation in an environment wherethe ambient temperature was 0° F. and the relative humidity was 60%. Noheating or cooling was provided in heat exchanger 42. Air stream 1entered turbine 20 at 0° F. and 60% relative humidity, was compressed incompression section 34 and was combusted in combustion section 36 by228.1 MMBTU per hour of a low heating value gas in the presence of 9696pounds per hour of NO_(X) water. The exhaust stream was expanded inexpansion section 38 and generated shaft work which was transmitted toelectrical generator 22 by shaft 24 to generate 23,560 kilowatts ofelectricity. Exhaust stream 5 exited turbine 20 at 869° F. and at a rateof 165.5 pounds per second. Exhaust stream 5 generated 82,800 pounds perhour of 150 psig saturated steam in steam generator 28 by vaporizingcondensate return available at 200° F. and make-up water. Exhaust gasstream 5 exited steam generator 28 at 306° F. No cooling or heatingmedium was circulated via stream 11 and no steam was utilized from steamline 7.

EXAMPLE 4

The gas turbine of Example 3 was placed in the same environment as inExample 3 at an ambient temperature of 0° F. and 60% relative humidity.Air stream 1 was heated in heat exchanger 42 to 35° F. and 18% relativehumidity by hot water which circulated therethrough at the rate of 800gallons per minute with an inlet temperature 100° F. and an outlettemperature of 75° F. via stream 11. The heated air stream 1 wascompressed in compression section 34 and combusted in combustion section36 by 238.9 MMBTU per hour of low heating value gas in the presence of10,785 pounds per hour of water. The exhaust gases were expanded inexpansion section 38. The shaft work that was transmitted to electricgenerator 22 via shaft 24 generated 24,378 kilowatts of electricity.Exhaust stream 5 exited turbine 20 at 964° F. at a rate of 160.2 poundsper second and generated 86,700 pounds per hour of 150 psig saturatedsteam in steam generator 28 by evaporating condensate return availableat 200° F. and make-up water. Exhaust stream 5 exited steam generator 28at 288° F. The circulating water was heated from 75° F. to 100° F. inheating source 30 by 9,900 pounds per hour of steam available fromstream 7 and flowing to heating source 30 via streams 13 and 16.Examples 3 and 4 show that the preheating of air stream 1 from 0° to 35°F. increased the output of turbine from about 23,560 to about 24,378kilowatts.

While a preferred embodiment of the present invention has been shown anddescribed, various modifications of the apparatus and the process of theinvention may be made by one skilled in the art without departing fromthe spirit of the invention and it is to be understood that theinvention is limited only as defined in the following claims.

What is claimed is:
 1. An apparatus, comprising:a multi-shaft combustion gas turbine having a first inlet for receiving air, a second inlet for receiving fuel, and an outlet for exhausting hot gases from the turbine; and a heat exchanger for flowing the air therethrough to heat the air before entering the first inlet to increase the power output of the turbine.
 2. An apparatus according to claim 1 wherein the turbine is a double shaft turbine.
 3. An apparatus according to claim 1 wherein the turbine is a triple shaft turbine.
 4. An apparatus according to claim 1 further including:means for supplying heat to the heat exchanger; an electrical generator; and a shaft for transferring work from the turbine to the electrical generator to generate electricity.
 5. An apparatus according to claim 1 further including:a steam generator; means for flowing water to the steam generator; and means for flowing the hot gases exiting the outlet of the turbine to the steam generator.
 6. An apparatus according to claim 1 further including means for transferring heat from the exhaust gases exiting the outlet of the turbine to the heat exchanger.
 7. An apparatus, comprising:a multi-shaft combustion gas turbine having an inlet for receiving air, the power output of the turbine being affected by the temperature of the air entering the inlet so that it reaches its maximum at an optimum temperature of the air entering the inlet and decreases as the temperature of the air entering the inlet decreases below or increases above the optimum temperature; and means for heating the air entering the inlet to increase the temperature of the air towards the optimum temperature if the temperature of the air is less than the optimum temperature.
 8. An apparatus according to claim 8 further including means for cooling the air entering the inlet to decrease the temperature of the air towards the optimum temperature if the temperature of the air is more than the optimum temperature.
 9. An apparatus according to claim 7 further including means for providing a heating medium to the heating means.
 10. An apparatus according to claim 8 further including means for providing a cooling medium to the cooling means.
 11. An apparatus, comprising:a multi-shaft combustion gas turbine having an air inlet for receiving air for use by the turbine, the temperature of the air affecting the power output of the turbine; and a heat exchanger for raising or lowering the temperature of the air flowing to the air inlet by exchanging heat between the air and a heating or a cooling medium to maximize the power output of the turbine.
 12. An apparatus according to claim 11 further including means for providing cooling to the heat exchanger to cool the air.
 13. An apparatus according to claim 11 further including means for providing heat to the heat exchanger to heat the air.
 14. An apparatus, comprising:a combustion gas turbine wherein the air inlet temperature of the air flowing into the turbine has an effect on the power output of the turbine, the effect being characterized by a maximum power output at an optimum air inlet temperature and a decrease as the air inlet temperature increases above or decreases below the optimum air inlet temperature; and a heat exchanger for flowing the air therethrough prior to flowing into the turbine to cool the air, if the temperature of the air is above the optimum air inlet temperature or the heat the air, if the temperature of the air is below the optimum air inlet temperature.
 15. An apparatus according to claim 14 further including:means for supplying cooling to the heat exchanger to remove heat from the air flowing therethrough, if the temperature of the air is above the optimum air inlet temperature; and means for providing heat to the heat exchanger to add heat to the air flowing therethrough, if the temperature of the air is below the optimum air inlet temperature. 